Nickel-hydrogen rechargeable batteries with an anode containing a hydrogen storage alloy provide higher energy density compared to nickel-cadmium rechargeable batteries, and pose less environmental burden in the absence of poisonous Cd. Nickel-hydrogen rechargeable batteries have been used in portable appliances such as digital cameras, and electric tools, as well as in electric vehicles and hybrid-type electric vehicles, and demanded to provide various battery characteristics depending on their applications.
As conventional anode materials for nickel-hydrogen rechargeable batteries, there have been used a LaNi5 hydrogen storage alloy, which is a rare earth-Ni intermetallic compound having the CaCu5 crystal structure as the main phase, a hydrogen storage alloy containing Ti, Zr, V, and Ni as the constitutional elements and having Laves crystal structure as the main phase, or the like alloys.
Rare earth-Mg—Ni hydrogen storage alloys have recently been put into practical use, and nickel-hydrogen rechargeable batteries employing the alloys as the anode material are known to have high capacity.
Patent Publication 1 proposes a hydrogen storage alloy represented by a general formula AMx (1), where A is at least one element selected from IIA group and IIIB group, and M is at least one element selected from VB group, VIB group, VIIB group, VIIIB group, IB group, IIB group, IIIA group, IVA group, and VA group, and x meets the relationship of 2.7<x<3.8, wherein an average atomic radius r (in angstrom) meets the relationship 1.36≦r≦1.39 (2), and x in the general formula (1) and r meet the relationship 1.41≦0.017x+r≦1.45 (3). The publication discloses that this hydrogen storage alloy has a high hydrogen storage capacity, and a nickel-hydrogen rechargeable battery employing the hydrogen storage alloy in its anode is excellent in cycle life.
Patent Publication 2 proposes a hydrogen absorbing alloy represented by formula RE1-xMgxNiyAlzMa, wherein RE is at least one of Pr and Nd, M is an element other than a rare earth element, Mg, Ni, and Al, and x, y, z, and a fulfill the relationship of 0.15≦x≦0.30, 2.8≦y≦3.9, 0<z≦0.30, and 3.0≦y+z+a≦3.6 by atomic ratio, and having an intensity ratio (IA/IB) of not smaller than 0.6, where IA represents an intensity of the highest peak in a range of 2θ=30° to 34° in the X-ray diffraction pattern using CuKα-radiation as the X-ray source and IB represents the intensity of the highest peak in a range of 2θ=40° to 44°. The publication discloses that this hydrogen absorbing alloy has excellent corrosion resistance, and a nickel-hydrogen rechargeable battery employing the hydrogen absorbing alloy in its anode is excellent in cycle life.
However, the nickel-hydrogen rechargeable batteries employing the hydrogen storage alloys disclosed in Patent Publications 1 and 2 do not fulfill both the initial activity and the cycle characteristics at the same time.
Patent Publication 1: JP-2000-228191-A
Patent Publication 2: JP-2004-263213-A